An image forming apparatus includes plural photosensitive members to be exposed by plural exposing units such as Light Emitting Diode (LED) heads, respectively while placing the photosensitive members and light emitting faces of the plural exposing units in close proximity. In such an image forming apparatus, each of the exposing units is made movable between an exposing position where the exposing unit is close to the corresponding one of the photosensitive members, and a retracted position where they are separated from each other, thereby enabling the photosensitive member to be easily attached to and detached from a body of the image forming apparatus. At the exposing position where the exposing unit is close to the photosensitive member, in order to perform accurate exposure on the photosensitive member, the positional relationship between the photosensitive member and the exposing unit has to be kept constant. JP-A-2003-112446 describes an image forming apparatus in which an LED head is held by a cover, and a positioning of the LED head with respect to the photosensitive member is conducted by fitting a positioning projection into a positioning hole formed in an image drum unit holding a photosensitive member.
However, even when the image drum unit and the LED head are positioned by using the projection and the hole as in JP-A-2003-112446, adjacent LED heads cannot be positioned with respect to each other to be parallel.